A crown of the aforementioned type is known for example from EP Patent Application No. 1 586 960 in the name of the Applicant. The crown essentially includes a cap delimited at the bottom by a circular skirt, which has a plurality of flutes on the external surface thereof for facilitating handling of the crown by the user. The circular skirt delimits a hollow cylindrical housing inside which the uncoupling mechanism is arranged. The uncoupling mechanism includes, among other things, a rigid ring, which has a plurality of notches on the inner periphery thereof for cooperating with one or more generally circular spring elements in order to achieve the uncoupling function. In fact, the notched ring is secured to the crown whereas the spring elements are secured to a pipe onto which is screwed a winding stem, which acts on the winding mechanism for the mainspring. Provided that the resistant torque opposed by the spring elements is less than a predetermined value, the spring elements are driven in rotation by the rigid ring under the effect of the user activating the crown. In turn, the spring elements drive the pipe on which they are mounted and thus the winding stem. When the resistant torque opposed by the spring elements becomes greater than the predetermined value, because the mainspring is completely wound, said spring elements will deform elastically in response to activation of the rigid ring and will escape therefrom by sliding. The result of this is that the crown activated by the user will rotate without transmission of any torque. The mainspring is therefore protected from any excessive winding which could damage it.
The top end of the pipe has a projecting portion, for example square-shaped and inscribed in a hole, which is also square, made in the springs via which said springs are secured to said pipe. Thus, when a rotational movement is imparted to the springs, the latter drive the pipe in rotation via its projecting portion.
With use, the crown described above has a problem. When the pipe is not perfectly aligned axially with the crown and a play thus appears between these two parts, the springs, rigidly coupled to said pipe, are no longer suitably centred relative to the notched rigid crown. When the notched ring is driven in rotation, the springs can then pass from their normal elastic deformation state, to a plastic deformation state in which they undergo irreversible damage. A substantial drop is then observed in the threshold torque value that determines the transition between the state in which the notched ring drives the springs in rotation and the state in which said notched ring slides over said springs without driving them, which consequently means that the user is no longer able to wind the mainspring in an optimum manner.